I have caught trout, bass to 6# and some large crappie (to 2 1/3 lb.) on Diamond Valley Lake, but I haven't seen any fly fishermen after stripers. The lake is considered a no-body contact domestic water supply by MWD, so they require their approval sticker on any private boat: the inspection process is free, but pre-mix 2 strokes are not allowed. The California Department of Fish and Game has a website of approved 2010 Licensed Fishing Guides, and several of them from the Riverside and Temecula/Escondito area list stripers as one of their guided species, so try checking with them....RAP
"You can observe a lot just by watching" (Yogi Berra)